Multidirectional sound system

ABSTRACT

There is disclosed a sound system for producing at least three and typically four sound outputs from respectively different directions from the listener wherein the sound content per se and the directional information are encoded on a conventional standardized two-channel record or on a transmission by a conventional two-channel broadcasting medium such as stereo FM. The system is maximally compatible with existing systems in the sense that reproduction on existing two-direction (stereo) or one-direction (mono) sound systems is completely satisfactory although, of course, the extent of directionality reproduction is limited by the inherent characteristics of such existing systems. In a typical example, the system provides for four directional sound inputs with equal, 90* separation around a circle. The four sound inputs are fed to two sound channels, for example stereo recording or transmission channels. Each of the four sound input channels is fed in part to each of the two stereo channels but the polarity and/or amplitude of each sound input channel is different in each of the stereo channels. Conventional analog computer electronic circuits may be utilized to transform the four sound input signals into two stereo channels according to prescribed formulae. The system provides for reproduction of sound from four loudspeakers located in the four corners of a room and having nominal positions with respect to the listener of left front, right front, left rear and right rear. The two stereo channels are combined according to different formulae setting forth different amplitudes and/or polarities to produce four output sound channels. Assuming these four directions correspond to directions of the four input sound channels, and sound originating from a particular sound input channel is reproduced predominantly in the corresponding loudspeaker. Refinements for the system control the gain for the respective loudspeakers to permit sound from a particular input sound channel to be localized to a particular corresponding output loudspeaker. In generalizations of the system, the number of inputs and the number of loudspeakers may be greater or less than four (but always more than two) and the numbers and/or directions of the input sound channels may not correspond to the numbers and directions of the outputs feeding the loudspeakers.

llnited States Patent [1 1 Scheiber MULTIDIRECTIONAL SOUND SYSTEM [76]Inventor: Peter Scheiber, 1987 Crompond Rd.,

Peekskill, NY. 10566 [22] Filed: June 15, 1970 [21] Appl. No.: 46,345

. Related U.S. Application Data [63] Continuation-impart of Ser. Nos.697,103, Jan. 11, 1968, and Ser. No. 853,822, Aug. 28, 1969, and Ser.No. 888,440, Dec. 29, 1969, Pat. No. 3,632,886.

[52] U.S. Cl 179/1 GQ, 179/100.4 ST [51] Int. Cl. H04lt 5/00 [58] Fieldof Search 179/1 G, 1 GP, 15 ST, 179/l00.4 ST, 100.1 TD

[56] References Cited UNITED STATES PATENTS 3,375,329 3/1968 Prouty179/1 G 3,401,237 9/1968 Takayanagi... l79/100.4 ST 2,019,615 11/1935Maxfield 179/1 G 2,335,575 ll/l943 Bierwirth 179/1 G 2,098,561 1111937Beers 179/1 G FOREIGN PATENTS OR APPLICATIONS 1,196,711 7/1965 Germany179/1 G Primary Examiner-Kathleen H. Claffy Assistant Examiner-ThomasDAmico Altorney- Darby and Darby [57] ABSTRACT There is disclosed asound system for producing at least three and typically four soundoutputs from respectively different directions from the listener whereinthe sound content per se and the directional information [451 July 17,1973 are encoded on a conventional standardized twochannel record or ona transmission by a conventional two-channel broadcasting medium such asstereo FM. The system is maximally compatible with existing systems inthe sense that reproduction on existing twodirection (stereo) orone-direction (mono) sound systems is completely satisfactory although,of course, the extent of directionality reproduction is limited by theinherent characteristics of such existing systems. In a typical example,the system provides for four directional sound inputs with equal, 90separation around a circle. The four sound inputs are fed to two soundchannels, for example stereo recording or transmission channels. Each ofthe four sound input channels is fed in part to each of the two stereochannels but the polarity and/or amplitude of each sound input channelis different in each of the stereo channels. Conventional analogcomputer electronic circuits may be utilized to transform the four soundinput signals into two stereo channels according to prescribed formulae.The system provides for reproduction of sound from four loudspeakerslocated in the four corners of a room and having nominal positions withrespect to the listener of left front, right front, left rear and rightrear. The two stereo channels are combined according to differentformulae setting forth different amplitudes and/or polarities to producefour output sound channels. Assuming these four directions correspond todirections of the four input sound channels, and sound originating froma particular sound input channel is reproduced predominantly in thecorresponding loudspeaker. Refinements for the system control the gainfor the respective loudspeakers to permit sound from a particular inputsound channel to be localized to a particular corresponding outputloudspeaker. In generalizations of the system, the number of inputs andthe number of loudspeakers may be greater or less than four (but alwaysmore than two) and the numbers and/or directions of the input soundchannels may not correspond to the numbers and directions of the outputsfeeding the loudspeakers.

14 Claims, 12 Drawing Figures United States Patent [1 1 Scheiber A R 2|ENCODER MATERIAL R22 SOURCE FROM ENCODER [451 July 17,1973

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INVENTOR. PETER SCHIEBER EM 4 Ya 45 ATTORNEYS PATENTED 3,746,792

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ATTORNEYS MULTIDIRECTIONAL SOUND SYSTEM This application is acontinuation-in-part of prior copending application Ser. No. 697,103filed Jan. 1 l, 1968 entitled STEREOPHONIC SOUND SYSTEM; Ser. No.853,822 filed Aug. 28, 1969 entitled STE- REOPl-IONIC RECORDING ANDTRANSMISSION SYSTEM (now abandoned); and application Ser. No. 888,440filed Dec. 29, 1969 entitled QUADRASONIC SOUND SYSTEM now US. Pat. No.3,632,886, all in the name of Peter Scheiber.

A complete system in accordance with the invention would includemultidirectional sound pickup apparatus, encoding apparatus forconverting the multiple channels from the pickup apparatus into only twoseparate electronic channels, recording apparatus and playback apparatus(or transmitting apparatus and receiving apparatus), decoding apparatusfor producing multidirectional sound electrical signals correlatingrespectively with the multidirectional sound input signals, and audioamplifiers and loudspeakers or equivalent apparatus for producing amultidirectional sound effect for one or more listeners.Multidirectional shall herein be interpreted to mean representative ofat least three directions of sound, and in a typical case fourdirections of sound. It is permissible and in fact highly desirable forcertain portions of the system described above to be strictlyconventional. For example, transmitting and receiving portions of asystem would typically consist of a conventional stereo FM broadcasttransmitter and an FM broadcast receiver.

For many years the enjoyment of stereo or bidirectional soundreproduction has been a common reality, at least in this country.However, such bidirectional sound reproduction apparatus has seriousinadequacies in reproducing music or other audio entertainment with aneffect approaching that obtainable with live performances.

The desirability of expanding the bidirectional sound techniques tomultidirectional sound techniques has been expounded in prior copendingScheiber patent applications and elsewhere. The desirability ofmultidirectional sound systems over bidirectional sound systems isintuitively apparent when one appreciates that the bidirectional soundsystem can only simulate sound sources existing within a limited angleof substantially less than 180, whereas a multidirectional sound systemcan simulate sounds originating from any direction, thus encompassing360. The augmented realism and impact of multidirectional sound overbidirectional sound is fully borne out by actual experience.

In theory, there is no obstacle to creating multidirectional soundsystems. Thus, a four-direction sound system for magnetic tape can beextrapolated in an obvious way from stereo tape systems. Instead ofusing two magnetic tracks on the magnetic tape, one uses four tracks. Ineffect, the stereo system which resulted by effectively doubling themonaural system is again doubled to produce a four-direction system.Such a fourtrack sound system, while feasible in theory, fails to solvenumerous practical difficulties. The essential difficulty is that onehas effectively doubled the entire system for reproduction of sound.Thus, if one wishes to transmit by radio, it would require two stereo FMradio stations rather than the one stereo FM station required tobroadcast bidirectional sound. For magnetic recording, twice as manyrecord or playback heads are required, twice as many tape tracks, andtwice as much tape. The difficulty in the case of disc recordings iseven greater since it appears that two grooves or tracks on the discwould be required, or else some complex subterfuge to obtain anequivalent recording capacity, neither of which has been found to bepractical.

In accordance with the present invention, it has been found thatsubstantially all the directionality information which can be reproducedby four loudspeakers and appreciated by a listener can be recorded andtransmitted without doubling and in fact ithout substantially increasingthe basic information-carrying capacity of two conventional audiochannels (for example those of a stereo record or stereo FM radiobroadcast).

In the system according to the invention, the directionality informationis encoded in the amplitude and phase (or polarity) relationships ofeach multidirectional input signal in one of the stereo channels ascompared to the other channel. While the directionality infonnation isnot recorded with a precision equal to that achieved for the frequencycomponents making up the audio information per se, this is quiteunimportant in view of the inadequacies of reproduction and evaluationof directional information in the system overall (including thelistener). By way of example, note that in any directional sound systemthere are a finite number of loudspeakers, and sounds emanating otherthan from those precise positions must be approximated by activatingloudspeakers in different positions with appropriate amplitudes. Thus asound which should emanate from directly in front of the listener mustbe approximated by sound from both a left front and a right frontspeaker.

In the present invention, the basic directional encoding and decodingarrangement utilizing amplitude and phase relation between the twostereo channels may be augmented by a gain control arrangement for theoutput signals. Such a gain control system permits the sound to belocalized in one loudspeaker to an extent greater than would be achievedwith the basic encoding and decoding system.

In a preferred embodiment, the invention also provides a system which ismaximally compatible with existing stereo FM broadcasting and withstereo disc recording. For example, if a disc record in accordance withthe present invention is played and the sound is reproduced from amonaural amplifier or monaural FM I radio receiver', or from a stereoamplifier or from a stereo FM radio receiver, the result is highlyacceptable monaural or stereo sound reproduction as the case may be. Thereproduction in either case includes all four sound inputs of therecording but with the rear sound input diminished in amplitude from theamplitude they would have if reproduced with a four-loudspeaker systemin accordance with the invention. No directionality distortion isintroduced in mono or stereo reproduction in the sense that the rightchannels and the left channels are produced equally in the monauralplayback and in the stereo playback are produced correctly in the bestpossible approximation to the four-speaker reproduction.

For simplicity of explanation, it is convenient to think in terms of amultidirectional sound system with four loudspeakers situated in thecomer of a substantially square room reproducing material having fourinput signals corresponding in direction to the four loudspeakersutilized in reproduction. Such an arrangement is encompassed in thepreferred embodiment of the system. However, a generalization of thesystem is presented such that the number of inputs to the encoder arenot limited to four, nor are they limited as to the direction which isto be represented by an output. In fact, the direction represented by aninput signal is determined by the values of certain resistors, and it isreadily possible to provide variable resistor networks such that thedirection represented by an input can be varied at will or can be setfor any desired direction. In a similar manner, the directionrepresented by the output loudspeakers can be changed from thepreviously described 90 separation to some other values. The directionassociated with a loudspeaker output may thus be changed to accommodatethe necessity of placing a speaker in an abnormal position. However, theposition represented by the loudspeaker output does not necessarily haveto conform to its physical position. Interesting effects can thereby beobtained. It will be seen that one can increase or decrease the anglewhich a distributed sound source such as an orchestra appears tosubtend. One can thus control the signal to the loudspeakers to createthe effect of moving from the rear of a hall where the orchestrasubtends a relatively small angle to the front of the hall (or in factthe podium) where the orchestra subtends a much larger angle.

The assignment of any desired direction to a loudspeaker output may alsoprove desirable for special situations such as automobile-installedsystems wherein the loudspeaker placement may best be other than theleft front, right front, left rear, right rear placement usual in aliving room or studio.

In addition to providing the advantages described above, it is an objectof the present invention to provide a multidirectional sound system suchthat records or transmissions in accordance with the system may beplayed on existing standard bidirectional or monodirectional equipmentwith completely satisfactory results comparable to recordings ortransmissions produced with only stereo or mono information content.

It is another object of the present invention to provide amultidirectional sound system wherein three or more sound input signalsare processed and transmitted over two channels, which two channelsignals are inversely processed to provide three or moremultidirectional output signals correlating to the input signals.

It is another object of the present invention to provide amultidirection sound system in which the assigned direction for soundinputs or outputs may be determined by the assignment of amplitudevalues as determined by resistor values or the like with the result thatthe effective direction assigned to an input sound signal or thatassigned to an output sound signal can be adjustably determined within awide angle.

Other objects and advantages will be apparent upon consideration of thefollowing description in conjunction with the appended drawings inwhich:

FIG. 1 is a diagram showing a typical speaker placement in relationshipto a listener and certain desirable phase or polarity relationships forapparatus according to the invention;

FIG. 2 is a schematic diagram of an encoder section of apparatusaccording to the present invention;

- FIG. 3 is a schematic diagram of a decoder and reproduction section ofapparatus according to the present invention;

FIG. 4 is a diagram useful in explaining directionality effects obtainedby a typical system in accordance with the invention;

FIG. 5 is a diagram useful in explaining the subjective efi'ect ofreproduction on conventional stereo reproduction equipment of fourdirection sound signals produced in accordance with the invention;

FIG. 6 illustrates the effect of reproduction on monaural reproductionequipment of four direction sound signals produced in accordance withthe invention;

FIG. 7 is a diagram illustrating the directional effect obtainable withan alternative form of system in accordance with the present invention;

FIG. 8 is a diagram illustrating the directional effect of afurtheraltemative system in accordance with the present invention;

FIG. 9 is a schematic diagram of a gain control arrangement for afour-directional system which may be adapted to decoding systems inaccordance with the invention to produce greater localization of soundfrom individual ones of the four output loudspeakers;

FIG. 10 is a schematic diagram of an alternative form of gain controlfor a four-directional sound system in accordance with the invention;

FIG. 11 is a schematic diagram of a gain control signal generator forproviding a low frequency gain control signal in recorded or transmittedmaterial to properly control the gain in apparatus as illustrated inFIG. 12; and

FIG. 12 is a schematic diagram of a gain control system adaptable foruse in conjunction with the described four-direction sound systemwherein the localization of sound output to various loudspeakers iscontrolled by a low frequency signal.

AMPLITUDES AND POLARITIES FOR ENCODING AND DECODING As previouslystated, the present system takes three or more directional sound inputsignals and combines them into two conventional audio informationchannels (such as utilized in stereo recording or broadcasting) in amanner to impart multidirectional sound direction information.

According to the invention, there are essentially two parameters to beconsidered in processing the multidirectional sound signals. These twoparameters are phase and amplitude. It is convenient to limitconsideration of phase relationships to only two different discretephase relationships, namely zero degrees (in phase) and (out of phase).These phase relations may also be treated as a simple reversal inpolarity or sign.

Considering first the amplitude relationships, it will be noted that onewishes to have a formula for determining the amplitude with which aparticular input sound signal is to be supplied to the A channel (leftchannel) of a conventional stereo recording or transmission system andthe amplitude with which such signal is to be supplied to the B channel(right channel) of the system.

It will be seen that the appropriate amplitude relation for the A and Bchannels can be determined as a function of the direction assigned tothe particular input signal. In this discussion, the direction assignedto the input signal will be identified by an angle x for signal No. l,x, for signal No. 2, and so on.

The angle of the position directly to the right of the listener isarbitrarily assigned the value of zero degrees and the angularprogression is counterclockwise. Hence the positions, illustrated inFIG. 1 for example, of right front, left front, left rear and right rearwould have angular position designations of 45, 135, 225 and 315respectively. See also FIG. 4.

Using this convention, the amplitude of an input signal supplied to theA channel is equal to the amplitude of the input signal multiplied bythe sine of one half the position angle. The equation for A signalamplitudes is given in Equation 1 of the Appendix hereinafter.

The amplitude of the signal supplied to the B channel is equal to theamplitude of the input signal multiplied by the cosine of one half theposition angle. The equation for the B channel signal amplitudes isgiven in Equation 2 in the Appendix.

It should be noted that the terms in Equations 1 and 2 all have apositive sign, but that this is not intended to indicate the polarity orphase of the signals. The polarity is determined in accordance with thequadrant in which the direction angle lies as explained hereinafter.

It is also necessary, of course, to define the appropriate decodingprocess to produce output signals correlating with the three or moreinput signals and having a desired directional characteristic. Theangular position convention for decoding is the same as for encoding.Each output signal g g etc. has an amplitude equal to the A channelamplitude times the sine of one half the position angle for the outputsignal plus the amplitude of the B channel times the cosine of one halfthe position angle for the output signal. The decoding equation is givenas Equation 3 in the Appendix. As before, the polarity or sign for thesignals is determined by the quadrant in which the position angle liesrather than by Equation 3.

An important aspect of the system is the manner in which the phase orpolarity relations for different angular position designations of outputand input are related to the A and B channel signal polarities. In orderto gain some intuitive understanding of the encoding and decodingcriteria described above, it is useful to note the operation of thesystem for a particular input signal.

Consider an input signal which is assigned the angular position of leftfront or 135. It can be shown that in accordance with the above encodingand decoding criteria, and assuming that the output loudspeakers haveangular positions of 45, 135, 225 and 315, the output correlating to thehypothetical input signal will be predominantly from the left front or135 loudspeaker.

There will be no output from the right rear speaker and there will besome output from the right front speaker and the left rear speaker. Ineach of the latter two speakers, the power output will be one half thepower (0.7 times amplitude) of the left front speaker level. The phaseor polarity of the signal from the right front speaker and from the leftfront speaker are desired to be the same as are the polarities of theleft front and left rear speakers. This is indicated in FIG. 1 by thenotation in on the lines joining the left rear and left front and theright front and left front speakers.

The use of positive and negative polarities in the system requires thattwo adjacent speakers, at least, must be of opposite polarity or out ofphase. According to the preferred embodiment of the invention, only thetwo rear speakers have this out-of-phase condition, this being mostcompatible with the prevalent situation in which the principal subjectmaterial rarely originates predominantly from the rear speakers.

Another important consideration is that in standardized FM stereo-radiobroadcasting, a monaural receiver reproduces a signal for the listenercorresponding to the sum of the A channel and the B channel in equalamplitude. Referring to the decoding relation ship, it will be seen thatthe position angles which yield equal A and B amplitudes are and 270.One desires that the monaural radio receiver have a position anglecorresponding to front center or 90. Hence, the A channel and B channelmust be combined with the same polarity or in phase for the front centeror 90 position. This also means that a four-direction broadcast receivedon a stereo FM receiver would produce left front sounds and right frontsounds from the left and right speakers respectively which are in phase(of the same polarity). This is obviously the desired situation. Inother words, for one-direction and two-direction playback compatibility,the present invention provides that the front center direction berepresented by A equal to B and of the same polarity, that is, theposition angle x equals 90 and A is at the left and B is at the right.

In reproduction of four-directional sound, the most important directionis obviously front center. Due to the predetermined arrangement ofspeakers in the four corners, the sound image for a front centerdirectional sound input is necessarily a ghost image between the frontpair of speakers. It is important that in addition to the front speakershaving equal amplitude for this situation, that they be in phase. Such asituation prevails in FIG. 1. It is also desirable that this frontcenter sound input, to the extent that it is reproduced in the rearspeakers, be reproduced in the same polarity or in phase. It will beseen in FIG. 1 that this situation also prevails.

The situation with respect to those images existing between a frontspeaker and a corresponding rear speaker is not so critical but it isdesirable that any outof-phase or opposite polarity reproduction of suchsignal be in an opposite rear speaker and that is also shown in FIG. 1.

As previously mentioned, one wishes to have any ghost image existingbetween two speakers produced in phase in each of the flanking speakersand this situation is maintained to the maximum extent possible as shownin FIG. 1 (excepting only the ghost images in the rear quadrant).

It will be seen that a particular assignment of polarities or phaserelationships for the encoding and decoding formulas must be observed toachieve the desirable reproduction relationships depicted in FIG. 11.Referring to Equations 1, 2 and 3, the proper polarities can besummarized as follows. All sine terms should be assigned a positivevalue for left front, right front and left rear quadrants, and anegative value for the right rear quadrant. All cosine terms should beassigned a positive value for the left front, right front and right rearquadrants and a negative value for the left rear quadrant. The foregoingpolarities or signs are in lieu of the sign for the trigonometricfunction for the particular quadrant. It is of interest that thedifference between the assigned value and the trigonometric values fromzero through 360 is only different in the fourth quadrant, i.e. theright rear. If one assigns the right rear quadrant angle values of fromminus 90 to zero rather than 270 to 360, then the algebraic sign of thetrigonometric functions would correspond to the desired values asderived from the analysis illustrated in FIG. 1.

ENCODER APPARATUS Suitable apparatus for encoding in accordance with thesystem herein described is shown in FIG. 2. It must be noted that theparticular form the apparatus takes is subject to great variation. Aswill be seen from Equations 1 through 3, the operation to be performedis quite simple in that it involves multiplying respective audiofrequency inputs by predetermined constants and adding or subtractingthe products so obtained to derive an encoded A channel or a B channelsignal. Numerous forms of conventional and readily available electronicanalog computer circuits or components may be utilized to perform thisoperation.

Referring to FIG. 2, multidirectional signals f f f and f, are obtainedfrom a multisignal source 11. The multisignal source would typicallyconsist of a multiple track tape recording produced at a recordingsession with various microphones or other audio input devicesrepresenting sound input signals to be assigned angular position valuesfor multidirectional sound reproduction. The angular position valueassigned to a particular recorded sound signal may correspond to anactual direction from a listener position in an actual recordingsession. The assigned angular position value for a particular soundsignal can, however, just as well be purely arbitrarily assigned toproduce a desired directional sound effect.

A schematic circuit diagram for the encoder is shown within the dashedoutline box 12.

The encoder circuit is shown with four inputs for separate and distinctsound input signals but it should be noted that more inputs may beprovided, and the number of inputs is not determined by the number ofoutput loudspeakers (which would normally be four in number). It will benoted that with more than four inputs, two or more inputs may beassigned to the same quadrant or to the same position angle resulting inthe encoder serving the function of a mixing and sound effect controlapparatus as well as an encoder. One sound signal may also be suppliedto two encoder inputs for specific effects.

The function of the encoder of FIG. 2 is to process the signals f f fand f, in accordance with Equations 1 and 2 to arrive at output signalsA and B.

A pair of operational amplifiers l3 and 14 together with appropriateinput resistors, feedback resistors and other resistors utilized in awell-known manner are employed to derive the output signals A and B. Theamplifiers 13 and 14 may, for example, be Philbrick'Nexus 101 lamplifiers which have the advantage of being capable of driving 600 ohmcharacteristic impedance output lines directly. Numerous otheramplifiers may also be used, noting of course that they must havesuitably high gain over the full audio frequency band for which theencoder is intended. Fifteen cycles to fifteen thousand cycles wouldnormally be ample frequency coverage.

Resistors Rl through R8 in FIG. 2 are input resistors, the resistancevalues of which may conveniently be utilized to determine the positionangle which is to be assigned to each of the four multidirectional inputsignals. The circuit illustrated in FIG. 2 is intentionally selected sothat it is not limited to a prescribed set of position angles for therespective input signals. Rather, a formula has been derived whichpermits the position angles to be set at different values over a widerange. Since certain position angles are associated with negativepolarity and other position angles are associated with positive polarityfor the sine and cosine terms of Equations 1 and 2, a limit is imposedto some extent on the assignment of position angles to the respectiveinputs. Accordingly, signal f and signal f may be located anywhere inthe first or second quadrants. Signal f may be located anywhere in thethird quadrant, and signal f, may be located anywhere in the fourthquadrant.

It may also be noted that for circuit economy, the amplifiers l3 and 14are provided with three inputs to their negative input terminal (notcounting the feedback) and only a single input to the positive inputterminal. One could also simply provide all inputs to the amplifier tothe one negative input terminal and utilize an inverter amplifier inseries in any of the inputs which one desired to provide with anopposite polarity.

In the circuit of FIG. 2, resistors R9 and R11 are feedback resistors,resistors R10 and R12 are ground resistors, and resistors R13 and R14are trimming resistors. Resistors R15 and R16 are output resistors.

In accordance with well-known analog computer techniques, the values ofresistors R9 through R12 are selected to make circuit values fall withina convenient range and to make input impedances sufficiently high toavoid loading associated circuits. These values would usually be in therange between ten thousand and several hundred thousand ohms.

The values of resistors R13 and R14 are selected in accordance withoperational amplifier manufacturer instructions to null DC offset.Resistors R15 and R16 are small isolating resistors to prevent loadingfrom effecting operational amplifier stability and may be of the orderof thirty ohms. The values of resistors R1 through R8 depend upon theangular position assigned to corresponding inputs as set forth inEquations 4 through 11 in the Appendix.

As a specific example of an encoder circuit, one may utilize a basicconfiguration of input position angles as illustrated in FIG. 4. As seenfrom FIG. 4, input f is at input f is at 45, input j}, is at 225, andinput 1; is at 315. The equations for the amplitudes of the A channeland B channel signals are given in Equations 12 and 13, and the valuesfor resistors R1 through R16 are as follows:v

R9 l00.0k

R13,R14 are selected to cancel amplifier DC offset R15,R16 30 ohms Itmay be noted that if all (four or more) inputs to each operationalamplifier are to the negative terminal and inverters are provided forinputs desired to be of different sign, then simple relations (such asfor R1 and R) prevail for all input resistors and the two resistorsassociated with each input have values solely determined by that inputsposition angle. Variable resistors can thus be employed which arecalibrated to provide any desired position angle (at least over onequadrant, 90) for respective inputs.

DECODER APPARATUS FIG. 3 illustrates an exemplary decoder schematiccircuit diagram as part of the playback portion of the overall system.Similarly to the decoder schematic circuit diagram, the circuitillustrated in FIG. 3 is one way to implement Equation 3. Other knownelectronic analog computer circuits could also be used within the scopeof the invention.

In FIG. 3, an encoder material source produces two electrical signaloutputs representing an A channel and a B channel. The encoder materialsource could, for example, be a conventional stereo record playerplaying a record of material encoded by the apparatus of FIG. 2.Alternatively, the encoder material source could be in stereo FM radioreceiver receiving encoded material from a transmitter that had beeneither encoded from a live broadcast or had been encoded on atwo-channel tape or disc record for playback over the FM stereotransmitter. In any event, the audio signal channels A and B willtypically be standard stereo transmission channels.

The A and B outputs of the encoder material source are the sole inputsto the decoder circuit contained within the dashed box 22 in FIG. 3.

A plurality of operational amplifiers 23, 24, 25 and 26 are providedwhich respectively generate four directional outputs for fourdirectional loudspeakers.

The A channel and B channel inputs are supplied to the operationalamplifiers with a particular amplitude ratio and polarity relationshipdetermined by the position angle assigned to the particular operationalamplifier and its associated loudspeaker.

Input resistors R21 and R22 determine the amplitude ratios of thechannel A signal and channel B signal supplied to amplifier 23,resistors R23 and R24 serve this function with respect to amplifier 24,resistors R25 and R26 serve this function with respect to amplifier 25,and resistors R28 and R27 serve this function with respect to amplifier26.

R29, R30, R31 and R33 are feedback resistors. R32 and R34 are groundresistors and R35, R36, R37 and R38 are trim resistors, all selected andused in accordance with known electronic analog computer techniques.

Outputs g,, g g and g, from amplifiers 23, 24, 25 and 26, respectively,are each supplied to the corresponding one of four power amplifiers 27,28, 29 and 30.

Power amplifiers 27 through 30 feed respective loudspeakers 31 through34. Loudspeakers 31 through 34 are, of course, arranged for directionalsound effects, for example as illustrated in FIG. 1.

Amplifiers 27 through 30 and loudspeakers 31 through 34 may be ofconventional form. While illustrated .as a single loudspeaker, each ofthe loudspeakers 31 through 34 may'comprise a loudspeaker system andenclosure for improved audio reproduction. Similarly, amplifiers 27through 30 may have controls, indicators and other features normallyassociated with audio power amplifiers.

It will be noted that the inputs to amplifier 23 from channel A andchannel B are of the same polarity as is also the case with amplifier24. On the other hand, the B input to amplifier 25 and the A input toamplifier 26 are of the opposite polarity.

With the specific polarity arrangement illustrated, amplifier 23 has anoutput which may be assigned a position angle anywhere in the left frontor right front quadrant, and the same is true of amplifier 24. Theoutput of amplifier 25 may be assigned a position angle anywhere in theleft rear quadrant, and the output of amplifier 26 may be assigned aposition angle anywhere in the right rear quadrant. As previouslyexplained, the position angle assigned for the operational amplifieroutput feeding a particular loudspeaker may or may not correspond to theactual physical position of the loudspeaker in the listening room.

If the position angles for the loudspeakers 31, 32, 33 and 34 aredesired to correspond to the position angles illustrated in FIG. 1 (andthus to the input position angles illustrated in FIG. 4), the relativeamplitudes of stereo channels A and B in each of the operationalamplifier output signals g g g and g, are readily calculated fromEquation 3 in the Appendix, the results being given in Equations 14through 17 in the Appendix.

To instrument the Equation 14 through 17 for the FIG. 3'dec0der circuit,the following resistor values (in ohms) are appropriate:

R22 261.3k R23 261.3k R24 108.2k

R25 108.2k I R26 201.4k R27 108.2k

R23, R30, R31 and R33 100k R32 and R34 k R35, R36, R37 and R38 areselected to cancel amplifier DC offset. Amplifiers 23, 24, 25 and 26 maybe the same as those described with reference to FIG. 2. Resistors R30through R34 are selected to make circuit values fall in a convenientrange.

Referring now to the complete system shown and described in FIGS. 2 and3 where the position angles are as illustrated in FIGS. 1 and 4, theoperation can be described in fairly simple terms.

It can be shown that an input signal f appears with greatest amplitudein the output signal g, feeding amplifier 27 and loudspeaker 31. It alsoappears at a reduced level in output signal g, and output signal 83feeding loudspeakers 32 and 33 respectively. The level of output inspeakers 32 and 33 is reduced by one half in power (3dB) or in amplitudeto a level of 0.707. There is no output in signal g corresponding to aninput signal f,.

Generalizing from the full power, half power and zero power relationshipdescribed above for the apparatus of FIGS. 1 through 4, it can be statedthat output for a given input signal will be a maximum when the positionangle of the output signal is the same as the position angle of tbeinput signal. When the position angle of the output signal is difierent(by an angle dx) from the position angle of the input signal, the outputwill be reduced by multiplying its amplitude by a factor equal to thecosine of half the angle of difference. An expression for theattenuation in decibels S to which an input signal of a prescribedposition angle is subjected in a particular output signal is given inthe Appendix as Equation 18. This is also the separation in signal levelwhich is obtainable between two output signals having a difference inposition angle (d1) as prescribed.

It should be noted that while the position angle arrangement for inputsillustrated in FIG. 4 with inputs at 45, 135, 225 and 315 is convenientto use as the basis for a simple explanation, one should consider otherinputs at other position angles to better understand the operation ofthe system. An input at 90 or front center is of especial interest.

A front center input obviously will be fed to loudspeakers 31 and 32(left front and right front) equally and with slight attenuation (about0.7 dB).

A front center-input will also appear to a minor extent in outputs g andg (and in speakers 33 and 34). The level of this output is quite low,however, being attenuated by approximately 8 dB.

It should be kept in mind that the particular input and output positionangles, presented in FIGS. 1 and 4 as a specific example and basis fordescription and explanation, are not the only possible embodiments nornecessarily the best embodiment for all purposes. Of course, thefour-corner arrangement of loudspeakers as illustrated in FIG. 1 is aparticularly practical one.

MONO AND STEREO COMPATIBILITY Just as it is useful to consider thepossibility of different input position angles than those of FIG. 4, itis also useful to consider different output position angles than thoseillustrated in FIG. 1 and corresponding to FIG. 4.

This is especially useful in considering compatibility with existingstereophonic and monophonic audio equipment. This existing equipmentwill be seen to have a corresponding position angle or position anglesby which one can evaluate the monophonic and stereophonic reproductionof audio material encoded in accordance with the invention andparticularly in accordance with the apparatus illustrated in FIG. 2.

Considering first stereophonic reproduction, it is well known, ofcourse, that in such reproduction the stereo channel A is applied to aleft loudspeaker and the stereo channel B is applied to a rightloudspeaker (in stereo FM broadcast and reception, this occurs afternumerous intermediate steps).

Referring to Equation 3 for decoding in the Appendix, it may readily beseen that reproduction of the A channel signal alone without anycontribution from the B channel corresponds'to a position angle of 180(the cosine of one half of 180 is zero). Similarly, the reproduction ofthe B channel alone without any contribution from the A channelcorresponds to a position angle of zero degrees (the sine of one half ofzero degrees is equal to zero). From previous descriptions of theoperation of the system, it will then be clear that a conventionalstereo reproduction system will operate to reproduce in its leftloudspeaker the subject matter that would have been reproduced in a leftfront and left rear loudspeaker of a four-directional system inaccordance with the invention. Similarly, the right loudspeaker of aconventional stereo system would reproduce the material which would havebeen reproduced in the right front and right rear loudspeakers of afour-directional system.

Using the same approach to the reproduction of fourdirectional materialon monaural equipment, for example a monaural FM receiver receiving abroadcast from a stereo FM broadcast station, it will be noted that thein-phase and equal combination of the stereo A channel and B channelreproduced in such a system corresponds to a position angle or a frontcenter position (the sine of one half of 90 is equal to the cosine ofone half of 90). The effect on the listener of reproduction offour-directional material on either conventional stereo or conventionalmonaural equipment is schematically illustrated in FIGS. 5 and 6.

Referring to FIG. 5, a pair of speakers 51 and 52 are shown togetherwith indications of typical sound image locations for four-directionalmaterial received by stereo equipment. Note that the image for leftfront fourdirectional audio material is, in fact, at left front in thetwo-speaker reproduction and the right front image is at right front,and that the left and right materials are properly balanced. It will benoted that the image for left front material and for right frontmaterial is displaced inwardly somewhat from the speaker location. Thisproduces no material adverse effect, however, and is necessary so thatthe extreme speaker position available in the two-speaker system bereserved for the maximum right and left position angles of zero degreesand The rear material image from the left speaker 51 is indicated by aseries of small xs. This is a schematic indication of the fact that therear or reverberant channel material will be affected by a slightsubjective outward displacement and spreading efiect due to theoutof-phase or opposite polarity relation of the rear position anglematerial. This effect is entirely in keeping with the desired use ofthese position angles to create ambience. Accordingly, the two-directionreproduction (stereo reproduction) of the four-direction materialproduces some emphasis for the front direction yet does not lose therear direction material entirely. Thus reproduction on stereo equipmentis nearly exactly what one would choose to have and provides excellentcompatibility.

Referring to FIG. 6, a single speaker 53 is shown representing amonaural system speaker such as a monaural FM receiver speaker whichmight be tuned to a stereo FM broadcast transmitter. In suchcircumstances, the sound im'age obviously can only be in direct linewith the loudspeaker and the relative power levels for various materialwill be: left front and right front zero dB and balanced, left rear andright rear material substantially diminished (-7.6 dB) but stillpresent. Only the material encoded atexactly center rear (270) will beeliminated entirely in monaural pickup. For mono reproduction, verylittle rear material, if any, is desired. Mono listening is usually donein less than ideal or even rather noisy environments, and it isimportant that primary (front) direction information should beemphasized as in fact results from the fourdirection material monauralrepoduction.

In regard to both monaural and stereo reproduction of the four-directionmaterial, it should be noted that considerable control is exercised overthe way that the material will be reproduced in stereo or mono byselection of position angles for the inputs to the encoder. Being awareof exactly how the material would be affected in stereo or monoreproduction, one may arrange that the monaural and stereo reproductionis of excellent quality as well as the four-directional reproduction,which is of course the main objective.

ENCODING WITH ALTERNATIVE POSITION ANGLE DESIGNATIONS FOR INPUTS FIG. 7shows alternative direction angle inputs which are particularly suitablefor certain musical material to be encoded in accordance with themultidirectional system of the present invention.

In situations such as the concert hall, where the front encoder inputswill be required to carry the most critical primary program information,while the rear encoder inputs supply ambience, or reverberation, one maywish to enhance the separation between the front encoder inputs at theexpense of reducing the separation for the rear encoder inputs. This maybe accomplished, for example as shown in FIG. 7, by prescribing agreater difference in position angle (dx) between the front pair ofinputs and less between the rear pair of inputs.

As shown in FIG. 7, x equal 150, x equal 30, x equal 240 and equal 300.The resulting encoding equation for channel A and channel B is given inthe Appendix as Equations 19 and 20. The encoder resistor values forFIG. 2 which would provide the input position angles shown in FIG. 7 aregiven below.

R2= 386.4k R3 115.5k

R4 259.1k R5 386.4k R6 103.51:

R7 115.5k R8=259.lk

R33 386.4k R34 103.5k R35 115.5k R36 136.6k R37 115.5k

R42, R44 50.0k

R15,R16,R17,Rl8 are selected to cancel amplifier DC offset The conceptof separating the position angles for either the input or the output inthe system to an angle greater than 90 for the front input or outputsignals is, of course, not limited to the particular position angledesignations illustrated in FIG. 7. A limiting case for such separationis illustrated in FIG. 8 in which the front position angles have beenspread to 180. The

rear position angles have been set with a negligible separation. Theposition angles of FIG 8, therefore, are for signal 1, 180; for signal2, 0; for signal 3, 269; for signal 4, 271. It is to be noted thatsignal 3 is still in the third quadrant and has the polarityrelationships designated for that quadrant, while signal 4 is in thefourth quadrant and has the polarity relationships designated for thatquadrant. If the extreme case shown in FIG. 8

were used as position angle designations for a decoder output where thespeakers were located in the four corners of a room, the left frontspeaker would have the A channel alone supplied to it, the right frontspeaker would have the B channel alone supplied to it, the left rearspeaker would have the A channel minus the B channel at a reducedamplitude, and the right rear speaker would have the B channel minus theA channel at a reduced amplitude. Thus the two rear speakers would havethe same content except for being out of phase with each other. Whilethe arrangement of FIG. 8 would not likely actually be employed in itsexact extreme version, it well illustrates the manner in which the leftfront and right front channel speaker separations can be increased toany desired extent by designation of appropriate decoder output positionangles.

Note that for greatly distorted position angle designations as in FIG.8, some adjustment in signal levels in indicated. For example, the tworear speakers in FIG. 8 are reproducing rear center material so thevolume should be decreased accordingly (or one speaker omitted).

It should further be noted that notwithstanding the current preferencefor placement of loudspeakers in the four corners of a room or studio sothat their relationship to the listeners is left front, right front,left rear and right rear, it is perfectly feasible to arrange afourdirectional system with speakers arranged in positions left, right,front and rear or in the centers of the walls of a listening room orstudio. In such an arrangement, the position angles for both the inputand output may correspond with the loudspeaker location and thus theposition angles would be 0, 90, land 270. The encoding equations forsuch an arrangement are given in Equations 21 and 22 in the Appendix,and the decoding equations are given in Equations 23 through 26.

It will be noted that Equations 21 through 26 correspond to encoding anddecoding equations in prior copending Scheiber patent applications.

It is obvious that numerous circuits other than those illustrated inFIGS. 2 and 3 can be used to provide the functions of the encoder anddecoder. For example, suitable arrangements of operational amplifiers ortranformers or combinations thereof may be used in either or bothdevices to provide the required functions. It is possible for atransformer type decoder to be located preceding or following the poweramplifiers. In the latter case, only two power amplifiers are requiredfor all four channels which means that existing stereo systems can beadapted merely by adding the decoder and two speakers at the receivingor playback station.

GAIN CONTROL APPARATUS The separation between adjacent speakers providedby the encoder and decoder embodiments of FIGS. 2 and 3 alone providesthe desired result, i.e. location of a virtual sound source at any placeon a circle around a listener. However, to further emphasize the effectin respect to highly localized sound sources, it may be desired toprovide substantially unlimited separation between adjacent speakers forsuch highly localized sounds. This can be accomplished in a number ofways, some of which are. of particular utility with the invention and,as such, may be considered to be an improvement over the basic system ofFIGS. l4. Four such improvements are described below with reference toFIGS. 9, 10, 11 and 12.

FIG. 9 is a block diagram of a simplified fonn of a gain control circuitwhich varies the gain of any pair of diagonal channels (e.g. left frontand right rear) with respect to the other diagonal channels.Hereinafter, by diagonal channels is meant the any pair of channelshaving an angular difference it substantially equal to 180 as definedelectrically and algebraically with reference to FIG. 1 but notnecessarily corresponding to the physical placement of the loudspeakers.

Since, in accordance with a basic embodiment of the invention, any giveninput will appear in three adjacent speaker channels with maximum gainin the speaker channel corresponding to the input channel, thedirectional effect can be emphasized by decreasing the gain of the twospeakers on either side of the desired speaker. In the system of FIG. 1,these two diagonal speakers are also placed in physically diagonalpositions. It can also be shown that where the absolute value of the LFsignal is equa o the absolute value of the RR signal (i.e. the wagetr msare identical except for possibly opposite polarity), the sound sourceshould either be located at the right front or left rear speaker. Thus,when this corfdition exists it is desirable that the gain for the rightfront and left rear signals be maximum relativeto the gain for the leftfront and right rear signals. Similarly, when either the RR or LP signalis zero or the waveforms in RR and LF are unrelated, the

sound source should be located at the left front or right 7 rearspeaker, in which case the gain for the right front and left rear shouldbe minimum relative to the gain of the left front and right rearsignals.

The foregoing shows that the separation between the outputs of FIG. 1(and thus the directional characteristics of the audio output) can beemphasized by simultaneously varying the gain in each pair of twodiagonal outputs alternatively to controlling the gain in each of theindividual channels. It is also desirable that the gain in one pair ofdiagonal channels be accompanied by an appropriate decrease in the gainof the other diagonal channels. Otherwise, an increase in gain (forexample) to enhance the directional characteristic of a signal wouldresult in a volume change of the total audio output as a function ofdirection. However, by simultaneously decreasing the power gain in onepair of diagonal channels (e.g. from 1 to 0) while increasing the powergain in the other diagonal channels (e.g. from 1 to 2), it is possibleto maintain the total power at the speakers constant, and separationbetween adjacent speakers can be increased without changing the totalvolume of the system. These functions are performed by the systemsillustrated in FIG. 9.

In FIG. 9, the decoder output is shown at the left. The four signals LF,RR, RF and LR are coupled to respective variable gain amplifiers 110LF,RR, RF and LR, which provide the signals for driving the four speakersas indicated. The LF and RR channels are also coupled directly throughhigh-pass filters IIZLF and ll2RR to absolute value circuits 114LF and114RR. The absolute value circuits 114 may comprise full-wave rectifiersthe outputs of which are of the same polarity. These signals,representing the absolute value of the LF and RR signals, are fed torespective logarithmic amplifiers 116LF and 116RR, which are well-knowndevices, providing output voltages approximately equal to the logarithmof the applied input voltage over the usable signal voltage range. Theoutputs of these amplifiers 116AF and 116BR are coupled through partialsmoothing filters 117A and 1178 to the negative and positive inputs,respectively, of an. operational amplifier 118 which subtracts the twosignals providing an output substantially dependent on log ILF l logIRRI (i.e. log

| LF/RR] This signal is then fed through another absolute value circuit120 and an averaging network 122 (an integrating circuit) to a gaincontrol generator 123 which controls the gain of the two pairs ofvariable gain amplifiers 110LF, 110RR and MORE 110LR as a function ofthe output of amplifier 118.

As indicated above, where the absolute values of the LF and LR signalsare equal, the gain of amplifiers 110 RF and l 10LR should be maximumand the gain of amplifiers 110LF and 110RR a minimum. When thiscondition exists, the output from the operational amplifier l 18 will beequal to zero and the power gain of amplifiers 110RF and 110LR should bea maximum (e.g. two) while the gain of amplifiers 110LF and (RR is aminimum (e.g. zero). At the other extreme, where either the RR or LFsignal is equal to zero or the waveforms in RR and LF are unrelated, theoutput of the amplifier 118 will be a maximum (theoretically infinitebut limited in practice to a definite value, for example, 9 volts). Thismaximum voltage causes the gain control generator 123 to provide outputvoltages which maximize the gain of amplifiers ll0LF and 110RR andminimize the gain of amplifiers 110RF and 110LR.

For conditions between those described above, the gains of therespective pairs of amplifiers 110 will be appropriately controlled bygenerator 123. Mathematically, it can be shown (assuming a FIG. 4embodiment of the system) that the curve of the required gainapproximates a square root curve to yield constant total acousticalpower output, with the gains in the respective diagonal channels beingequal when the amplitude ratio of LF to R (of RR to LF) is about 2.4 andthe waveforms are the same. The equations 27 and 28 in the Appendix maybe used to determine the (amplitude) gain control voltages fromgenerator 123 where T is the time constant of the averaging circuit 122,K is a constant, r is time, V is the RF and LR control voltage and V isRF-LR control voltage.

One purpose of the high-pass filters 112A and 1128 is to prevent thepassage of low-frequency signals which might otherwise appear on theinputs to the variable gain amplifiers 110 and possibly modulate theamplifier inputs. It has further been found desirable to discriminateagainst lower frequency signals (at 6 dB per octave). The filters 112Aand 1128 also serve this function.

Filters 1 17A and 1 178 may have time constants from to l ,000microseconds and their outputs are in pan responsive to the envelope oftheir inputs and in part responsive to instantaneous values. Each typeof response is preferred indifferent situtations and thus a desirablecompromise is achieved by filters 117A and 1178. The averaging circuit122 should respond to changes in the output of the amplifier 118 quicklyenough so that the ear does not notice the delay, but

not so fast as to pass the actual waveform to the amplitiers 110. As anexample, a 20 millisecond charging rate has been found satisfactory forpractical purposes. to stabilize the action of the gain control circuitsof the decoder, it may be desirable to mix slightly the LF and RRsignals at the encoder output (or the left and right inputs to theencoder) to minimize excursions of the LF/RR signal ratio. Conversely,to prevent the log of this ratio from going to zero, constant phasedifferences may be introduced between the respective signals applied tothe A and B channels at the encoder. This has the effect of restrictinggain control action to a relatively narrow range so that the gaincontrol action will not be audible at the speakers. Such mixing can bedone in proportions which will accomplish the desired result withoutmaterially altering the audio characteristics. Where extreme channelseparation is required, this technique would not be used.

As noted previously, there are many different ways of controlling thegain in the respective channels to provide the desired directionalenhancement at the speakers. The embodiment illustrated and describedwith reference to FIG. 9 is a relatively inexpensive way of providingthe desired gain.

FIG. shows an alternative gain control arrangement in which the gainassociated with each speaker is determined by a combination of a gaincontrol element serially connected in the respective speaker input, anda gain control voltage generator whose output is coupled to the gaincontrol element. The audio signal in each decoder output passes throughthe respective gain control element. Then, according to the outputsignal of the control voltage generator, the signal in the gain controlelement is either enhanced or attenuated. When the output signal of thecontrol voltage generator is at a maximum, the output of the gaincontrol element is at a maximum, and vice versa.

The gain control elements 203 and 204 are thus controlled by an outputvoltage V produced by the control voltage generator 210. The gaincontrol elements 208 and 206 are controlled by an output voltage Vproduced by the control voltage generator 212. If desired, separatecontrol voltage generators may be respectively coupled to each gaincontrol element.

The expressions for each of the control voltages V and V are dictated bydesign considerations of the various control voltage generators whichproduce these expressions, as well as by the specific phase, waveform,and level cues present in the original signals A and B which are toactivate the respective speakers.

For example, a desired acoustical reproduction requires that the gainassociated with the speakers 31 and 34 increases as the ratio of theintensity levels of the signals g, and g, diverges from unity, or theirwaveforms become increasingly dissimilar. To achieve this result, thecontrol voltage V, applied to the gain control elements 203 and 204 maybe represented by one of various expressions.

V, may be proportional to the average absolute value of the logarithm ofthe quotient of the absolute values of g, and 3,. Alternatively, V, maybe proportional to the average absolute value of the logarithm of thequotient of the sum and difference of the absolute values of g, and g Asa third alternative, V, may be proportional to the average of thequotient of the sum and differ ence of the absolute values of g, andg.,. Equations 29-3l in the Appendix represent some expressions forV,.In addition to the above or in combination with the above, similarexpressions may be employed in which the envelopes of g, and g, aresubstituted for the instantaneous signals.

The gain for channels 32 and 33 must obviously be varied in acomplementary manner to that of speakers 31 and 34 so that the voltage Vfrom control voltage generator 212 may be proportional to the constantminus the expression for the voltage V,. Control voltage V, increases asthe loudness level associated with either the g, or g, signals becomesstronger with respect to the other, or their waveforms becomeincreasingly dissimilar.

The gain for speakers 32 and 33, on the other hand, is to increase asthe ratio of the intensity levels of each of the signals g, and gapproaches unity and as their waveforms become similar.

GAIN CONTROL SIGNAL APPARATUS FIGS. 11 and 12 illustrate a furtherembodiment of a gain control system employing the basic principles ofthe invention wherein subsonic control tones are impressed upon the Aand B channels for the purpose of controlling the gain of the two pairsof diagonal channels from the decoder 20. In describing the operation ofFIGS. 11 and 12, the microphones, speakers, encoder and decoder performthe same function as previously described and therefore are notdescribed further.

To facilitate an understanding of this embodiment it is convenient torefer to power ratios rather than voltage ratios as previously. Thepower which can be de rived from a given signal is directly proportionalto the square of the voltage level of that signal.

The signal recording means is illustrated in FIG. 11. The outputs of theright front and left rear microphones 2RF and ZLR are sensed and coupledto a poweradding circuit 130, while a similar power-adding circuit 131sums the power outputs from microphones ZLF and 2RR. These twopower-adding circuits are devices which produce output voltages directlyproportional to the total power which can be derived from the appliedinput voltages. Their output voltages are then summed in an addingcircuit 132, the output of which is thus proportional to the total powerin the four input channels.

The outputsof summing circuits and 1132 are coupled to a ratio circuit134 which, in turn, causes respective A and B modulators I36 and 138 tomodulate a 20- cycle (or other subsonic) tone from oscillator M0. Ratiocircuit 134 may be any of a number of wellknown circuits and, forexample, may produce a direct output voltage having an amplitudeproportional to the ratio of the applied input voltages.

Modulators 136 and 138 may be adapted to amplitude-modulate the 20-cycletone from oscillator 11410 with respect to a preselected level,depending upon the magnitude of the applied control voltage from theratio circuit 134. When the A modulator 136 provides a tone of increasedamplitude, the B modulator I133 should be providing a tone ofproportionately decreased amplitude. These modulated tones are thenadded to the A and B outputs of encoder 18 to provide the signals whichare to be conveyed by the two-channel transmission path and which, inthis particular embodiment, are indicated as A and B.

The receiving end of the system is illustrated in FIG. 12. Two high-passfilters 142 and 144 are used to separate the audio control tones fromthe A and B audio signals on the A and B channels. These A and B signalsfrom the filters 142 and 144 are coupled to decoder to provide the fouroutput channels described above.

The control tones from the filters 142 and 144 are coupled to a gaincontrol generator 146 which controls the gain of variable gainamplifiers 148RR, RF, LF and LR to increase the gain of one pair ofdiagonal channels while appropriately decreasing the gain of the otherpair of diagonal channels. From the preceding discussion of FIG. 11, itfollows that the amplitudes of the control tones will each be equal tothe desired power in their corresponding diagonal input channels dividedby the total power in the system. Each of these signals varies from avalue of zero to one and their sum should always equal one. Accordingly,since the desired power ratios (i.e. the power ratios at themicrophones) are directly represented in the control tone signals it isa simple matter for gain control generator 146 to utilize these knownratios to control the gain of amplifiers 148RR, LF and 148LR, RF torecreate the same ratios at the outputs of the amplifiers 148. This willnecessarily enhance the desired signals while deemphasizing thesesignals which are not in their corresponding channels. The total powerwill also not be varied due to directionality changes. Generator 146also serves as a normalizer to maintain the total gain of the fourchannels such that the sum of the power in the respective channels ismaintained equal to a constant. This prevents unwanted changes in theamplitude of the control tone from affecting the volume of the outputsfrom the respective loud-speakers.

From the foregoing description of various embodiments ofmultidirectional sound systems in accordance with the invention, it willbe seen that a highly effective system is provided capable ofreproducing virtually all essential directional information withoutsacrificing fidelity, frequency response or other qualities of r theaudio information. Also transmission or recording is possible using onlytwo conventional stereo channels. it should be appreciated that theparticular apparatus disclosed is not intended to represent a suitabledesign for manufacturing economy but is rather presented for ease ofexplanation and to show the manner in which the system can be assembledfrom well-known existing electronic analog computer compnents andcircuits. In practice, more economical transistor circuits would besubstituted for the expensive operational amplifier components, theresistors also would be accorded much more tolerance in resistancevalues than indicated in the description, and other practical economieswould be effected.

The gain control features described here are a useful adjunct to thesystem, but are not in all cases necessary. Furthermore, numerous othervariations of gain control systems to enhance the separation betweenspeakers or the localization of sound direction could be utilized otherthan the particular ones described here or in copending patentapplications.

More elaborate gain control systems could utilize analysis circuitssimilar to the ones described herein but duplicated or triplicated sothat each analysis circuit would serve to analyze a difi'erent frequencyband within the overall audio frequency band of the system.

Gain controls for adjustment of the front to rear power ratio (ratherthan the diagonal pair power ratio) are also potentially useful. Suchadjustment can be made on the basis that inequalities between front andrear power should generally be increased to tend to restore the powerratios to those of the input signals.

In addition to those variations and modifications to the systemdescribed or suggested herein, numerous other variations andmodificationswill be apparent to those skilled in the art. The inventionis to be understood not to be limited to the specific illustrationsdescribed and rather is to include those variations and modificationswithin the ordinary skill of the art.

APPENDIX A= sin x,/2 +f sin x /2 +f,, sin x,,/2 B=f cos x,/2 +f cos x /2+f,, cos x,,/2 g =A sin x,,/2 B cos x,,/2 R1 [R9/sin (x,/ 2)] R2 [R9/sin(x ,2)] R3 {R9/sin (X3/2)] R4 l sin (x,/2) sin (x /2) sin (x /2) l/ "whms/ 8.,R5 [R1 l/cos(x,/2)] 9. R6 [R1 1/cos(x /2)] 10. R7 [R1 1/cos(x/2)] ll. R8 l cos (x /2) cos (x /2) cos (x /2) 608 41 m/co a/ H 12. A0.9239f 0.3827f 0.9239f 0.38271 13. B 0.3827f 0.9239fi 0.3827f 0.9239f,14. gl 0.9239A 0.38278 l5. g2 0.382724 0.92398 16. g3 0.923911 0.3827817. g4 0.3827A 0.92393 18. S 20 log cos (dx/Z) 19. A 0.9659}; 0.2588f,0.8660 0.5000 20. B 0.2588f 0.9659f 0.5000f 0.86601" 21. A =f 0.707 010722. B 0107 0.70m 23. gl A 24. 32 B 25. g3 0.7071! 0.7078 26. g4 0.707110.7078

LF V) if -i 1 1 4 K T log RR d: (27) LF V K 1 f 10 g R 8) 29. V] K G4)30. V, K log (G -(M (id-(i What is claimed is:

1. In a multidirectional sound system for encoding at least threedirectonal input sound signals on A and 8 audio channels and reproducingfrom the A and B channels at least three directional output soundsignals correlated with the input signals, encoder apparatus comprisingat least three inputs for input sound signals having respective positionagnles associated therewith, first means for generating an A channelsignal connected to at least three of said inputs, said first meanscausing the amplitude of each said input sound signal in said A channelto be substantially proportional to the cosine of one half the angulardifference between the input position angle and the angle assigned tothe A channel, second means for generating a B channel signal connectedto at least three of said inputs, said second means causing theamplitude of each said input sound signal in said B channel to besubstantially proportional to the cosine of'one half the angulardifference between the input position angle and the angle assigned tothe B channel, the angles assigned to said A and B channels differing byapproximately 180 degrees, decoder apparatus comprising an A channelinput and a B channel input, means for communicating said A channel andB channel signals to said decoder apparatus inputs, first, second andthird means for generating first, second and third directional soundoutput signals each having a respective position angle associatedtherewith, each said means being connected to each of said A and Binputs and causing the amplitude of each of said A and B inputs in saidoutput sound signal to be substantially proportional to the cosine ofone half the angular difference between the output position angle andthe angle assigned to the respective A or B input.

2. Apparatus as claimed in claim 1 wherein said A channel and B channelgenerating means cause the polarity of at least one of said input soundsignals in said A channel to be opposite to its polarity in said Bchannel.

3. Apparatus as claimed in claim 1 wherein said first, second and thirdmeans for generating output signals cause at least one of said A and Bchannel signals in one of said outputs to be opposite to its polarity inat least one other of said outputs and the same as its polarity in atleast another of said outputs.

4. Apparatus as claimed in claim 3 wherein said A channel and B channelgenerating means cause the polarity of at least one of said input soundsignals in said A channel to be opposite to its polarity in said Bchannel.

5. In a multidirectional sound system for encoding at least threedirectional input sound signals on A and B audio channels andreproducing from the A and B channels at least three directional outputsound signals correlated with the input signals, encoder apparatuscomprising at least three inputs for input sound signals havingrespective position angles associated therewith, first means forgenerating an A channel signal connected to at least three of saidinputs, said first means causing the amplitude of each said input soundsignal in said A channel to be substantially proportional to the cosineof one half the angular difference between the input position angle andthe angle assigned to the A channel, and second means for generating a Bchannel signal connected to at least three of said inputs, said secondmeans causing the amplitude of each said input sound signal in said Bchannel to be substantially proportional to the cosine of one half theangular difference between the input position angle and the angleassigned to the B channel, the angles assigned to said A and B channelsdiffering by approximately I80 degrees.

6. Apparatus as claimed in claim 5 wherein said A channel and B channelgenerating means cause the po' larity of at least one of said inputsound signals in said A channel to be opposite to its polarity in said Bchannel.

7. In a multidirectional sound system for encoding at least threedirectional input sound signals on A and B audio channels andreproducing from the A and B channels at least three directional outputsound signals correlated with the input signals, decoder apparatuscomprising an A input and a B input, at least three directional soundsignal outputs, first, second and third means for generating first,second and third directional sound output signals each having arespective position angle associated therewith, each said means beingconnected to each said input and causing the amplitude of each saidinput in said output sound signal to be substantially proportional tothe cosine of one half the angular difference between the outputposition angle and the angle assigned to the respective A or B input,the angles assigned to said A and B inputs differing by approximatelydegrees.

8. Apparatus as claimed in claim 7 wherein said first, second and thirdmeans for generating output signals cause at least one of said A and Bchannel signals in one of said outputs to be opposite to its polarity inat least one other of said outputs and the same as its polarity in atleast another of said outputs.

9. In a multidirectional sound system for encoding at least threedirectional input sound signals on A and B audio channels andreproducing from the A and B channels four directional output soundsignals correlated with the input signals, decoder apparatus comprisingan A input and a B input, first, second, third and fourth means forgenerating first, second, third and fourth directional sound outputsignals each having a respective position angle associated therewith, atleast three of said means being connected to each said input and causingthe amplitude of each said input in said output sound signal to besubstantially proportional to the cosine of one half the angulardifference between the output position angle and the angle assigned tothe respective A or B input, the angles assigned to said A and B inputsdiffering by approximately 180 degrees.

10. In a multidirectional sound system for encoding at least threedirectional input sound signals on A and.

B audio channels and reproducing from the A and B channels fourdirectional output sound signals correlated with the input signals,decoder apparatus comprising an A input and a B input, first, second,third and fourth means for generating first, second, third and fourthdirectional sound output signals each having a respective position angleassociated therewith, each said means being connected to each said inputand causing the amplitude of each said input in said output sound signalto be substantially proportional to the cosine of one half the angulardifference between the output position angle and the angle associated tothe respective A or B input, the angles assigned to said A and B inputsdiffering by approximately 180 degrees.

11. In a multidirectional sound system for encoding at least threedirectional input sound signals on A and B audio channels andreproducing from the A and B channels four directional output soundsignals correlated with the input signals, decoder apparatus comprisingan A input and a B input, first, second, third and fourth means forgenerating first, second, third and fourth directional sound outputsignals, each having a respective position angle associated therewith,each said means being connected to each said input and causing theamplitude of each said input in said output sound signal to besubstantially proportional to a function of the angular differencebetween the output position angle and the angle assigned to therespective A or B input.

12. In a multidirectional sound system for encoding at least fourdirectional input sound signals on A and B audio channels andreproducing from the A and B channels at least four directional outputsound signals correlated with the input signals, decoder apparatuscomprising an A input and a B input, at least four directional soundsignal outputs, first, second, third and fourth means coupled to said Aand B inputs for generating first, second, third and fourth directionalsound output signals each having a respective position angle associatedtherewith, each said means causing the amplitude of each said input insaid output sound signal to be substantially proportional to the cosineof one half the angular difference between the output position angle andthe angle assigned to the respective A or B input, the angles assignedto said A and B inputs differing by approximately 180.

13. Apparatus as claimed in claim 12 wherein said first, second, thirdand fourth means for generating output signals cause at least one ofsaid A and B channel signals in one of said outputs to be opposite toits polarity'in at least one other of said outputs and the same as itspolarity in at least another of said outputs.

14. In a multidirectional sound system for encoding at least threedirectional input sound signals on A and B audio channels andreproducing from the A and 8 channels four directional output soundsignals correlated with the input signals, decoder apparatus comprisingan A input and a B input, first, second, third and fourth means coupledto said A and B inputs for generating first, second, third and fourthdirectional sound output signals each having a respective position angleassociated therewith, each said means causing the amplitude of each saidinput in said output sound signal to be substantially proportional to afunction of the angular difference between the output position angle andthe angle assigned to the respective A or B input.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. v 2 vDa June 17, 1973 Inventor-(s) PETER SCHEIBER It is certified that errorappears in the above-identified patent and that said Letters Patent arehereby corrected as shown below:

On the cover sheet insert The portion of the term of this patentsubsequent to Jan. 4, 1989, has been disclaimed.

Signed and sealed this 21st day of January." 1975..

(SEAL) Attest:

MCCOY M. GIBSON JR. c. MARSHALL DANN Attesting Officer Commissioner ofPatents USCOMM-DC 60376-P69 u.s sovzmmznv nm'nms OFFICE: 8 6 9 93 o

1. In a multidirectional sound system for encoding at least threedirectonal input sound signals on A and B audio channels and reproducingfrom the A and B channels at least three directional output soundsignals correlated with the input signals, encoder apparatus comprisingat least three inputs for input sound signals having respective positionagnles associated therewith, first means for generating an A channelsignal connected to at least three of said inputs, said first meanscausing the amplitude of each said input sound signal in said A channelto be substantially proportional to the cosine of one half the angulardifference between the input position angle and the angle assigned tothe A channel, second means for generating a B channel signal connectedto at least three of said inputs, said second means causing theamplitude of each said input sound signal in said B channel to besubstantially proportional to the cosine of one half the angulardifference between the input position angle and the angle assigned tothe B channel, the angles assigned to said A and B channels differing byapproximately 180 degrees, decoder apparatus comprising an A channelinput and a B channel input, means for communicating said A channel andB channel signals to said decoder apparatus inputs, first, second andthird means for generating first, second and third directional soundoutput signals each having a respective position angle associatedtherewith, each said means being connected to each of said A and Binputs and causing the amplitude of each of said A and B inputs in saidoutput sound signal to be substantially proportional to the cosine ofone half the angular difference between the output position angle andthe angle assigned to the respective A or B input.
 2. Apparatus asclaimed in claim 1 wherein said A channel and B channel generating meanscause the polarity of at least one of said input sound signals in said Achannel to be opposite to its polarity in said B channel.
 3. Apparatusas claimed in claim 1 wherein said first, second and third means forgenerating output signals cause at least one of said A and B channelsignals in one of said outputs to be opposite to its polarity in atleast one other of said outputs and the same as its polarity in at leastanother of said outputs.
 4. Apparatus as claimed in claim 3 wherein saidA channel and B channel generating means cause the polarity of at leastone of said input sound signals in said A channel to be opposite to itspolarity in said B channel.
 5. In a multidirectional sound system forencoding at least three directional input sound signals on A and B audiochannels and reproducing from the A and B channels at least threedirectional output sound signals correlated with the input signals,encoder apparatus comprising at least three inputs for input soundsignals having respective position angles associated therewith, firstmeans for generating an A channel signal connected to at least three ofsaid inputs, said first means causing the amplitude of each said inputsound signal in said A channel to be substantially proportional to thecosine of one half the angular difference between the input positionangle and the angle assigned to the A channel, and second means forgenerating a B channel signal connected to at least three of saidinputs, said second means causing the amplitude of each said input soundsignal in said B channel to be substantially proportional to the cosineof one half the angular difference between the input position angle andthe angle assigned to the B channel, the angles assigned to said A and Bchannels differing by approximately 180 degrees.
 6. Apparatus as claimedin claim 5 wherein said A channel and B channel generating means causethe polarity of at least one of said input sound signals in said Achannel to be opposite to its polarity in said B channel.
 7. In amultidirectional sound system for encoding at least three directionalinput sound signals on A and B audio channels and reproducing from the Aand B channels at least three directional output sound signalscorrelated with the input signals, decoder apparatus comprising an Ainput and a B input, at least three directional sound signal outputs,first, second and third means for generating first, second and thirddirectional sound output signals each having a respective position angleassociated therewith, each said means being connected to each said inputand causing the amplitude of each said input in said output sound signalto be substantially proportional to the cosine of one half the angulardifference between the output position angle and the angle assigned tothe respective A or B input, the angles assigned to said A and B inputsdiffering by approximately 180 degrees.
 8. Apparatus as claimed in claim7 wherein said first, second and third means for generating outputsignals cause at least one of said A and B channel signals in one ofsaid outputs to be opposite to its polarity in at least one other ofsaid outputs and the same as its polarity in at least another of saidoutputs.
 9. In a multidirectional sound system for encoding at leastthree directional input sound signals on A and B audio channels andreproducing from the A and B channels four directional output soundsignals correlated with the input signals, decoder apparatus comprisingan A input and a B input, first, second, third and fourth means forgenerating first, second, third and fourth directional sound outputsignals each having a respective position angle associated therewith, atleast three of said means being connected to each said input and causingthe amplitude of each said input in said output sound signal to besubstantially proportional to the cosine of one half the angulardifference between the output position angle and the angle assigned tothe respective A or B input, the angles assigned to said A and B inputsdiffering by approximately 180 degrees.
 10. In a multidirectional soundsystem for encoding at least three directional input sound signals on Aand B audio channels and reproducing from the A and B channels fourdirectional output sound signals correlated with the input signals,decoder apparatus comprising an A input and a B input, first, second,third and fourth means for generating first, second, third and fourthdirectional sound output signals each having a respective position angleassociated therewith, each said means being connected to each said inputand causing the amplitude of each said input in said output sound signalto be substantially proportional to the cosine of one half thE angulardifference between the output position angle and the angle associated tothe respective A or B input, the angles assigned to said A and B inputsdiffering by approximately 180 degrees.
 11. In a multidirectional soundsystem for encoding at least three directional input sound signals on Aand B audio channels and reproducing from the A and B channels fourdirectional output sound signals correlated with the input signals,decoder apparatus comprising an A input and a B input, first, second,third and fourth means for generating first, second, third and fourthdirectional sound output signals, each having a respective positionangle associated therewith, each said means being connected to each saidinput and causing the amplitude of each said input in said output soundsignal to be substantially proportional to a function of the angulardifference between the output position angle and the angle assigned tothe respective A or B input.
 12. In a multidirectional sound system forencoding at least four directional input sound signals on A and B audiochannels and reproducing from the A and B channels at least fourdirectional output sound signals correlated with the input signals,decoder apparatus comprising an A input and a B input, at least fourdirectional sound signal outputs, first, second, third and fourth meanscoupled to said A and B inputs for generating first, second, third andfourth directional sound output signals each having a respectiveposition angle associated therewith, each said means causing theamplitude of each said input in said output sound signal to besubstantially proportional to the cosine of one half the angulardifference between the output position angle and the angle assigned tothe respective A or B input, the angles assigned to said A and B inputsdiffering by approximately 180* .
 13. Apparatus as claimed in claim 12wherein said first, second, third and fourth means for generating outputsignals cause at least one of said A and B channel signals in one ofsaid outputs to be opposite to its polarity in at least one other ofsaid outputs and the same as its polarity in at least another of saidoutputs.
 14. In a multidirectional sound system for encoding at leastthree directional input sound signals on A and B audio channels andreproducing from the A and B channels four directional output soundsignals correlated with the input signals, decoder apparatus comprisingan A input and a B input, first, second, third and fourth means coupledto said A and B inputs for generating first, second, third and fourthdirectional sound output signals each having a respective position angleassociated therewith, each said means causing the amplitude of each saidinput in said output sound signal to be substantially proportional to afunction of the angular difference between the output position angle andthe angle assigned to the respective A or B input.